Even though it's nice to use posix_spawn() instead of manually using
fork()/exec(), it's better to disable this. FreeBSD 7 doesn't support
this interface. When enabled, we can't build tblgen, which prevents us
from building FreeBSD 9 on 7.
Tested by: raj
I've looked at other places in the source tree where CLANG_VENDOR is
used and I suspect it might not be safe to use newlines here.
CLANG_VENDOR should just be defined to "FreeBSD ", just like the latest
Clang preview in OS X uses "Apple ". Properly use SVN_REVISION to define
it to the imported revision of Clang. I do want to have a date in there,
so slightly modify the code to support CLANG_VENDOR_SUFFIX.
